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Machband Effect

The what? The luminance of the squares above increases in a stepwise fashion. However although the luminance within each block is constant the apparent lightness of each strip seems to vary across its length. Close to the left edge of the strip it appears lighter than at the centre, and close to the right edge of the strip it appears darker than at the centre. The visual system is exaggerating the difference in luminance (contrast) at each edge in order to detect it. Try holding a pencil over an edge in this image and see how the apparent difference in lightness of two adjacent strips changes. They become much harder to tell apart. The edge detection is working to enhance object separation.